The present invention relates to a rubber composition suitable for reinforcing bead portion of pneumatic tires, and more particularly to a rubber composition suitable for use in a chafer in the bead portion of heavy duty tires for an auto truck, auto bus and other large vehicles.
Tires used for trucks, buses and so on are required to have high speed performance because of the spread of a superhighway, and in addition so that they can bear a larger load with the increase of size and loadage of the vehicles. Consequently, rubber compositions used in the chafer disposed in bead of tires are required more and more to have high hardness, high durability and high aging resistance.
Since these tires are recapped and reused by re-covering their tread rubber after the completion of the first life, they are also required to be free from fault. One of faults which prevents the recapping of used tires is deformation of the bead in tube tires. Bead deformation is a phenomenon that occurs in tube tires with the rubber in the bead caves in at a spacing between the wheel rim and a ring, thus resulting in deformation after use for a long term. Rubber compositions used in the chafer and bead of tube tires are required to have high hardness, high durability and high aging resistance also in order to prevent occurrence of this phenomenon.
In order to obtain rubber compositions satisfying these properties, it has been practiced (1) to select the kind of carbon black or to increase the amount of carbon black, (2) to increase the amount of sulfur, or (3) to use ultraaccelerator TT (tetramethylthiuramdisulfide). However, the use of a large amount of carbon black causes problems in processing that it becomes difficult to conduct milling with good dispersion, and burning takes place due to heat generation resulting from high loading, and a problem that the crack growth resistance of the rubber is deteriorated. When a large amount of sulfur is incorporated in a rubber, there arise problems that change in physical properties owing to heat, change in physical properties with the lapse of time and a permanent set (deformation owing to heat) become large, and the aging resistance and a resistance to breaking such as cracking are deteriorated. Since the thiuram ultraaccelerator TT produces nitrosoamine which is a cancer causing substance and was found as a mutagen, its use is being prohibited. It is also proposed in Japanese Patent Publication Kokoku No. 57-20164 to incorporate a syndiotactic-1,2-polybutadiene short fiber into a rubber. However, the short fiber cannot be uniformly dispersed by application of a mechanical force and, therefore, desired physical properties are not obtained by incorporation of only such a short fiber.
It is an object of the present invention to provide a rubber composition suitable for reinforcing the bead of tires and having high hardness, high durability and high aging resistance.
A further object of the present invention is to provide a rubber composition suitable for use in the chafer in the bead of tube tires, which is excellent in processability such that it does not cause roll bagging and burning, and which is also excellent in hardness, durability and aging resistance and provides tires which avoid deformation in the bead, rim-chafing and cracking.
These and other objects of the present invention will become apparent from the description hereinafter.